Introduce a basically working rkv based MeiliDB

src/store/synonyms.rs

@@ -0,0 +1,23 @@
+pub struct Synonyms {
+    pub(crate) main: rkv::SingleStore,
+    pub(crate) synonyms: rkv::SingleStore,
+}
+
+impl Synonyms {
+    pub fn synonyms_fst<T: rkv::Readable>(
+        &self,
+        reader: &T,
+    ) -> Result<fst::Set, rkv::StoreError>
+    {
+        Ok(fst::Set::default())
+    }
+
+    pub fn alternatives_to<T: rkv::Readable>(
+        &self,
+        reader: &T,
+        word: &[u8],
+    ) -> Result<Option<fst::Set>, rkv::StoreError>
+    {
+        unimplemented!()
+    }
+}